Effects of various concentrations of tetraethyl lead (TEL) on exhaust emissions were investigated. Eight Ford Galaxies equipped with production non-exhaust emission equipped 289-CID, 2-V engines were operated in customer service for 18,000 miles in the Detroit area. Four fuel blends were used in mileage accumulation — Indolene Clear (a full boiling range non-lead gasoline), Indolene 5 (Indolene Clear + 0.5 ml TEL/gallon in a motor mix blend*); Indolene 15 (Indolene Clear + 1.5 ml TEL/gallon in a motor mix blend), and Indolene 30 (Indolene Clear + 3.0 ml TEL/gallon in a motor mix blend). All engines were operated on the same petroleum base engine lubricant.Exhaust emissions were monitored at approximately 3,000 mile intervals using the CMVPCB** seven-mode procedure. Additional seven-mode tests were obtained on each engine after combustion chamber deposits were removed at the completion of the mileage accumulation phase.The increase in exhaust hydrocarbon (HC) emissions of vehicle operation on the leaded fuels was substantially higher than the HC increase of vehicles operated on Indolene Clear. At the 12,000 mile point, for example, the average HC increase for the leaded fuels was approximately 100 ppm as Hexane while the HC increase on Indolene Clear was negligible.No deleterious effect, due to fuel used, was observed on any engine component.